Ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focused on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. For example, US 2011/312050 describes a continuous process for the catalytic generation of polyols from cellulose, in which the cellulose is contacted with hydrogen, water and a catalyst to generate an effluent stream comprising at least one polyol.
CN 102643165 is directed to a catalytic process for reacting sugar in an aqueous solution with hydrogen in the presence of a catalyst in order to generate polyols.
As with many chemical processes, the reaction product stream in these reactions comprises a number of desired materials, diluents, by-products and other undesirable materials. In order to provide a high value process, the desirable product or products must be obtainable from the reaction product stream in high purity with a high percentage recovery of each product and with as low as possible use of energy and complex equipment.
In known processes to make glycols, the glycols are usually present at high dilution in a solvent, typically water. The water is usually removed from the glycols by distillation. Subsequent purification of the glycols is then carried out by fractional distillation. This process can have high costs both in terms of capital and operational expenditure. Further, repeated heating or maintenance at raised temperatures in the distillation steps may also lead to decomposition of the desired glycol products.
When glycols are produced by hydrogenolysis of sugars, a mixture of glycols is produced. The main glycol constituents in the reaction product stream are monoethylene glycol (MEG), monopropylene glycol (MPG) and 1,2-butanediol (1,2-BDO). The separation of these glycols by fractional distillation is complicated due to the similarity in boiling points, particularly between MEG and 1,2-BDO (respectively 198 and 196.8° C.). Further, the isolation of a pure MEG overheads stream by fractional distillation from a mixture comprising MEG and 1,2-BDO is made impossible by the formation of a homogeneous minimum boiling azeotrope between MEG and 1,2-BDO at atmospheric pressure.
Degradation of the products at high temperatures prevents higher than atmospheric pressure being used for distillation.
U.S. Pat. No. 4,966,658 is directed to the separation of a mixture of 1,2-BDO and MEG using a process known as azeotropic distillation in which an azeotrope-forming agent is added to the mixture before distillation in order to facilitate separation. A similar process is described in U.S. Pat. No. 5,423,955 for the separation of 1,2-BDO and MPG. Azeotropic distillation can lead to an increase in relative volatility between the components but also leads to further process steps in order to remove the azeotrope forming agents.
Another azeotropic distillation method is described in US20130284584. This document describes the use of an azeotropic agent of structural formula:
wherein each R group is hydrogen or an alkyl group.
Co-pending application EP 14163242.2 discloses a process for separating monoethylene glycol from a mixture comprising monoethylene glycol and 1,2-butanediol, using a two column, pressure-swing distillation set-up.
It would be advantageous to provide a simple and efficient method suitable for the recovery of MEG from a mixture comprising MEG and 1,2-BDO.